This invention relates to a novel method for improvement of printing characteristics of offset printing plates made utilizing the silver complex diffusion transfer process, especially, increase in the number of copies obtainable from the offset printing plate without any problems in images on halftone dots and fine lines and remarkable improvement in inhibition of occurrence of printing stains.
Direct use of a silver image obtained by the silver complex diffusion transfer process which is one of the photographic reproduction method as an offset printing master plate has been disclosed in Japanese Patent Unexamined Publication Nos. 43132/71, 30562/73, 55402/74, 106902/77 and 112402/77.
According to a typical embodiment of the silver complex diffusion transfer process suitable for making such lithographic printing plate, when a light-sensitive material comprising a support and, disposed successively thereon, an undercoat layer which serves also as antihalation layer, a silver halide emulsion layer, and a physical development nuclei layer is imagewise exposed and developed, the silver halide in the areas where a latent image has been formed changes into black silver in the emulsion layer. At the same time, the silver halide in the areas where a latent image has not been formed dissolves in a development processing solution or by the action of a complexing agent contained in at least one constituting layer as disclosed in Japanese Patent Examined Publication No. 84743/79 and diffuses towards the surface of the light-sensitive material. The dissolved and diffused silver complex is precipitated by the reducing action of the developing agent on the physical development nuclei in the surface layer to form a silver image. After the development, if necessary, the silver image is subjected to sensitization treatment to enhance its ink receptivity. The material thus treated is mounted on an offset printing machine and the inked image is transferred to a print material.
Various properties required for such printing plate depend on the printing materials and at the same time, depend strongly on plate making process, especially the development step. That is, the state of the transfer developed silver image is an important factor for effect on the printing characteristics and the state depends greatly upon the conditions under which the transferred silver grains are formed, namely, the diffusion speed, stability and reduction speed of silver complex. Printing plates made by development processing in accordance with the diffusion transfer process have the characteristics of high sensitivity, high resolving power and high image-reproducibility, but are inferior in printing endurance and image reproducibility to other commerciallized printing plates (e.g., PS plate) and Japanese Patent Examined Publication No. 1057/81 discloses a processing solution for such printing plates obtained as a result of studying silver complexing agents and additives by paying attention to the conditions for formation of transferred silver grains in the development processing such as, for example, diffusion speed, stability and reduction speed of silver complex in an attempt to improve both the printing endurance and the image-reproducibility. However, satisfactory printing endurance has not yet been obtained. Practically, reproducibility of the printing plate in terms of resolving power and printing endurance is about 500 copies in practical limitation of number of copies obtainable without problems in printing of 5% dot images of at least 200 lines/inch. Furthermore, when the thickness of a ruled line of original is less than 50 .mu.m, the ruled line reproduced on the printing plate becomes thinner due to scattering of light, this being unavoidable in case of reflective exposure and as a result, the silver on the plate disappears during printing and reproduction of the thin line becomes impossible.